This application relates to aircraft control systems and more particularly to aircraft sensor systems.
An aircraft is a vessel that is free to move in three dimensional space. FIG. 1 depicts a typical coordinate system useful for describing aircraft motion in three dimensions. In the body fixed coordinate system of FIG. 1, the aircraft has a longitudinal axis xb which extends along the length of the airplane. Rotation about the Xb axis, L, is called roll. The coordinate system of FIG. 1 further includes a lateral axis yb extending parallel to the aircraft wing. Rotation about the yb axis, M, is called pitch. The zb axis extends perpendicular to the remaining axes as shown. Rotation about the zb axis, N, is called yaw.
Equations of motion can be derived to describe the aircraft movement using the axes shown in FIG. 1. Unfortunately, the orientation and position of the aircraft in space cannot be truly understood with the coordinate system of FIG. 1 since the coordinate system is moving with and is always centered on the body of the aircraft. For this reason, it is common to transform the parameters of FIG. 1 to describe the angular displacement of the aircraft in space. These angular displacements, or Euler angles, are as shown in FIG. 2.
In good weather, under visual flight conditions, pilots of conventional aircraft control the aircraft motions and the resulting angular displacements in three dimensional space by visual reference to the natural horizon. The natural horizon serves as a visual clue from which the pilot can determine if the airplane is climbing, descending or turning. In low visibility conditions, such as, for example: nighttime, haze, or flight in clouds; the natural horizon can become obscured and the pilot is unable to control the aircraft by reference to the natural horizon. Conventional aircraft are therefore equipped with several instruments to assist the pilot in visualizing the aircraft""s movement in three dimensional space. These instruments also provide the pilot with supporting data from which to confirm control of aircraft even when the natural horizon is visible.
FIGS. 3A-3G show a conventional aircraft panel for a contemporary airplane having such standard instrumentation. The control panel of FIG. 3A includes: an altimeter 2 that provides the pilot with information on aircraft altitude; an airspeed indicator 4, that provides information on the aircraft speed through the air; and a vertical speed indicator 6, that provides data on the rate of climb and descent. Instruments 2, 4 and 6 comprise the pitot-static, or pneumatic, instruments since they operate by sensing air pressures exterior to the aircraft. In certain larger aircraft, the pitot static instrument sensors are combined into a single box called an air data computer. The air data computer then outputs the altimetry and airspeed data to a cockpit display and/or to other avionics equipment requiring such data.
Also included in the standard control panel of FIG. 3A are the gyroscopic instruments. The gyroscopic instruments provide the pilot with a pictorial view of the airplane""s rate of turn, attitude and heading. These instruments include a turn coordinator 8, an attitude indicator 10, and a heading indicator 12. A wet magnetic compass 13, may also be used to provide heading information. Wet compass 13 does not contain a gyro.
FIGS. 4A-4B illustrate aircraft turn coordinator 8 in greater detail. Turn coordinator 8 senses yaw, r, and roll, p, movement about the aircraft zb and xb axes. When the miniature airplane 14 is level as shown in FIG. 4A, the aircraft is neither turning nor rolling. When the aircraft banks, miniature airplane 14 also banks. In the drawing of FIG. 4B, miniature airplane 14 indicates a turn to the right.
FIGS. 5A-5D illustrate operation of aircraft attitude indicator 10 also known as an artificial horizon. Attitude indicator 10 senses pitching, "THgr", and rolling, xc3x8, movements about the airplane""s lateral and longitudinal axes. Attitude indicator 10 is the only flight instrument that provides both pitch and bank information to the pilot. Attitude indicator 10 presents a view of the aircraft, as represented by miniature airplane 20, as the aircraft would appear to someone standing behind it. The pitch attitude of the aircraft is shown by noting the position of the nose 22 of miniature airplane 20 relative to the artificial horizon 24. Bank information is shown both by noting the position of miniature airplane 20 relative to the deflected artificial horizon 24 and by the alignment of bank angle pointer 28 with the graduated bank angle indexes located on the perimeter of the device. FIG. 5A shows the aircraft in level flight and no turn. FIG. 5B shows the aircraft in a level turn to the left. FIG. 5C shows a level climb and FIG. 5D shows a descending left turn.
Heading indicator 12, also known as a directional gyro, serves as a means to indicate the aircraft magnetic heading without the limitations of using wet compass 13. Wet compass 13 is prone to various turning and acceleration errors. Heading indicator 12 is not subject to these errors and thus provides the pilot with a more stable indication of aircraft heading throughout the flight.
Each of turn coordinator 8, attitude indicator 10, and heading indicator 12 includes a gyroscope needed for proper operation of these instruments. Typically, the gyroscopes in attitude indicator 10 and heading indicator 12 are powered by a vacuum pump. Turn coordinator 8 is normally powered using an electric motor. The gyroscopes contained within each of these instruments also have operating limitations. For example, if the aircraft enters an extreme or unusual flight attitude, the gyroscope can tumble rendering the associated instrument inoperative.
Similar to the air data computer, the gyroscopic instruments are occasionally on larger aircraft combined into a single integrated sensor package called an attitude heading reference system, or AHRS. The AHRS system outputs the attitude data to a cockpit display and to other avionics equipment requiring such data.
In airplanes with autopilots, the autopilot uses the attitude information supplied by these gyroscopic instruments or AHRS instrument suite to fly the aircraft. Thus, when an instrumentation fault occurs, the autopilot is also affected.
Gyroscopic instruments are also prone to various types of errors during normal operations. FIG. 6 is a cut away view of a gyroscope and gimbal structure representative of those used in aircraft applications such as directional gyro 12. The gyroscope of FIG. 6 includes gyro wheel 30 mounted on an inner gimbal 36 which is in turn mounted on an outer gimbal 37. Directional gyro 12 and the heading gyro used in AHRS applications work best when the inner gimbal is exactly perpendicular to the outer gimbal. When the inner gimbal is correctly oriented, the gyro is said to be xe2x80x98erect.xe2x80x99 When the gyro is not erect, its output is inaccurate.
Modem gyros use an automatic erection system to maintain the gimbals in the proper orientation. In one such system, an inclinometer, in the form of a pendulum or an accelerometer is used. FIGS. 7A and 7B show a pendulum erection system used with vacuum driven gyros. In FIG. 7A, the pendulum 40 hangs in a first orientation when the gimbals are properly aligned. In FIG. 7B, pendulum 40 is displaced when the gimbals are not properly aligned, opening air vents 42 which then cause the gyro to move to the desired position under the force of the resulting air flow.
Accelerometer based systems work identical to the pendulum based systems. Both systems are sensitive to the acceleration of gravity and actual aircraft accelerations. Under steady state conditions, or with no aircraft accelerations, both (accelerometer and pendulum) will sense a proper vertical position or xe2x80x9clocally levelxe2x80x9d direction.
In some circumstances, the pendulum may cause opening of the air vents even though the gyro gimbals are correctly aligned. This condition can occur when the aircraft is in a steady state turn and the resultant gravity vector is now displaced to one side as shown in FIG. 8. The result is that the erection system will erroneously be activated causing the gyro to erect on a false vertical with correspondingly inaccurate output.
To counteract this problem, during turns greater than 6 degrees, the AHRS system deactivates the automatic erection system. This sends the gyro into a free drift mode during the turn. When the turn is completed, the angle measured using the gyro will have some small error. However, it is usually assumed that any resulting errors are small.
The present invention recognizes the problems associated with gyro cut out circuits used to prevent the gyro from erecting into a false vertical. According to one aspect of the present invention, a roll angle estimate based on existing aircraft yaw rate and true airspeed is used to slave the Euler roll attitude computation during periods when the bank angle causes the automatic erection system to place the gyro in free drift mode. In this manner, the system providing heading data to the pilot is supplied supplemental sensor data thereby reducing errors.
According to another aspect of the invention, the roll angle estimate may be output from a separate system used to provide roll angle, pitch angle and/or heading angle estimates when the primary AHRS or aircraft gyro system has a fault or failure. In such an embodiment, the invention includes a signal processing device further including logic, either software and/or hardware, for estimating aircraft roll angles, pitch angles and heading angles. Each angle estimator is coupled to a suite of sensors from which the desired angle may be estimated in the absence of sensor data directly measuring that angle. The sensors may be integrated with the present invention or located separately onboard the aircraft. Optionally, the roll angle estimator may be included integrally with the present invention.
Further details and operation of the invention are described below.